Steam generation through the combustion of fuels that contain nitrogen and sulfur lead to pollutants emissions such as nitrogen oxides (NOx) and sulfur oxides (SOx) to the atmosphere. NOx and SOx emissions to the atmosphere are known of having a negative impact on the environment. For example, NOx and SOx are responsible for acid rains and ozone depletion that affect the environment by reducing air quality and killing vegetation. They can also be responsible for serious troubles in human health in case of long exposure to these pollutants.
Due to their negative impacts as mentioned above, governments around the world monitor and control pollutant emissions, and impose restrictions for maximum levels of atmospheric emissions based on combustion plant size and type. Several processes are available for handling NOx and SOx removal from flue gases. These processes include precipitators to remove the dust and ash, catalytic or non-catalytic reactors to remove the NOx and dry or wet scrubbers to remove the SOx. This association of several technologies in order to remove the pollutants is usually very expensive in term of capital and operating costs since most of these technologies produce a by-product as a result of the flue gas cleaning. One way of reducing the cost of pollutant removal, is to integrate the flue gas cleaning into the boiler and reduce the amount of by-products produced by the flue gas treatment unit.
Climate change is another issue that is getting more attention, with particular focus on greenhouse gas emissions, since they are seen as the main culprit. Generally, the combustion of fossil fuels such as oil, coal or natural gas produces carbon dioxide (CO2), thereby adding to the greenhouse gas effect. Several technologies are under development to mitigate CO2 emissions. Among these technologies, CCS (CO2 Capture and Sequestration) is foreseen as one of the most efficient solutions to reduce CO2 emissions to the atmosphere. Oxy-combustion technology is part of CO2 capture technologies and is considered as one of the most economical route to capture CO2 for sequestration or Enhanced Oil Recovery (EOR).
Oxy-combustion technology uses oxygen (generally streams composed of more than 75% oxygen) instead of air to combust the fuel and produce a highly concentrated CO2 stream that is easier to capture in comparison to the conventional amine scrubbing technology. This can be achieved by removing nitrogen from air and consequently producing flue gas flow rate that is four to five times lower than conventional air combustion flue gas. In oxy-combustion, since the flue gas flow rate is very low, the concentration of SOx is high allowing for improved removal due to the higher SOx gradient. This can simplify the flue gas desulfurization unit and drastically reduce its cost. In addition, the removal of nitrogen from air suppresses the thermal NOx that are formed by the oxidation of nitrogen in air-combustion systems.
The removal of nitrogen from air will also increase the temperature of the flue gas in the combustion chamber, which drastically impacts the heat transfer. For example, in a conventional air-boiler where about 35% of the energy released in the combustion chamber is used to vaporize the water circulating in the walls, the flue gas temperature is about 1300° C. whereas the flue gas temperature in an oxy-boiler with the same energy extraction is about 3650° C. Obviously this temperature is not compatible with conventional materials used for designing boilers. To avoid this high level temperature, it is usually suggested to recycle part of the flue gas exiting the boiler to the combustion chamber in order to reduce the flue gas temperature to acceptable temperatures. This solution is useful, but has the main drawback of reducing the thermal efficient of the oxy-boiler since the recycled flue gas act only as temperature moderator.
It would be desirable to have an improved process for generating steam from the combustion of a sulfur-containing fuel with oxygen, while minimizing equipment size and reducing SOx and NOx emissions. It would be advantageous to regenerate the adsorbent used to remove sulfur products in order to recycle the adsorbent back into the system.